Changing Flutter Candidate to 3.18.0-0.0.pre (#6879)

Automated Flutter candidate version bump.

flutter-candidate.txt

diff --git a/flutter-candidate.txt b/flutter-candidate.txt
index aaec09a..5c336bd 100644
--- a/flutter-candidate.txt
+++ b/flutter-candidate.txt
@@ -1 +1 @@
-3.17.0-19.0.pre
+3.18.0-0.0.pre